cot 4600 operating systems spring 2011 dan c. marinescu office: hec 304 office hours: tu-th...
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COT 4600 Operating Systems Spring 2011
Dan C. Marinescu
Office: HEC 304
Office hours: Tu-Th 6:00-7:15 PM
Last time: Discussion of the paper “Architecture of Complexity” by H. A. Simon Modularity, Abstractions, Layering, Hierarchy
Today: Discussion of the paper “Hints for Computer Systems Design” by Butler
Lampson. Layering and Hierarchy for Coping with Complexity Complexity of Computer Systems Bandwidth and Latency Iteration Names and The Basic Abstractions Memory. Critical Properties of Memory Systems.
Read-Write Coherence Before or After Atomicity
Next time Raid Systems
Lecture 5 – Thursday, January 27 2011
Lecture 5
Layering for Coping with Complexity
Layering building a set of successive functional entities with restricted communication patterns, a layer may only communicate with the layer below it and with the one above it.
Examples: networking
Hierarchy for Coping with Complexity
Hierarchical structures construct a large system from a small collection of relatively large subsystems
Examples: Corporations An army A computer is a collection of subsystems
Names
Modularity along with abstraction, layering, and hierarchy allow a designer to cope with complexity;
Names and addresses provide the means to connect modules.
A system a bunch of resources, glued together with names
Naming allows the designer to: Delay the implementation of some modules; use dummy ones Replace an implementation with another one.
Binding choosing an implementation for a module Delayed binding; use a place holder.
Digital computers - a distinct species of complex systems
The complexity of digital computer systems not limited by the laws of physics distant bounds on composition Digital systems are noise-free. The hardware is controlled by software
The rate of change unprecedented The cost of digital hardware has dropped in average 30% per
year for the past 35 years
Lecture 5
Analog, digital, and hybrid systems Analog systems:
the noise from individual components accumulate and the number of components is limited
Digital systems: are noise-free the number of components is not limited regeneration restoration of digital signal levels static discipline the range of the analog values a device
accepts for each input digital value should be wider than the range of analog output values
digital components could fail but big mistakes are easier to detect than small ones!!
Hybrid systems e.g., quantum computers and quantum communication systems
Lecture 5
Computers are controlled by software
Composition of hardware limited by laws of physics. Composition of software is not physically constrained;
software packages of 107 lines of code exist Abstractions hide the implementation beneath module
interfaces and allow the creation of complex software modification of the modules
Abstractions can be leaky. Example, representation of integers, floating point numbers.
Lecture 5
Exponential growth of computers Unprecedented:
when a system is ready to be released it may already be obsolete. when one of the parameters of a system changes by a factor of
2 other components must be drastically altered due to the incommensurate scaling.
10 the systems must be redesigned; E.g.; balance CPU, memory, and I/O bandwidth;
does not give pause to developers to learn lessons from existing systems find and correct all errors
negatively affects “human engineering” ability to build reliable and user-friendly systems
the legal and social frameworks are not ready
Lecture 5
Coping with complexity of computer systems
Modularity, abstraction, layering, and hierarchy are necessary but not sufficient.
An additional technique iteration Iteration
Design increasingly more complex functionality in the system Test the system at each stage of the iteration to convince
yourself that the design is sound Easier to make changes during the design process
Lecture 5
Iteration – design principles
Make it easy to change the simplest version must accommodate all changes required by
successive versions do not deviate from the original design rationale think carefully about modularity it is very hard to change it.
Take small steps; rebuild the system every day, to discover design flaws and errors. Ask others to test it.
Don’t rush to implementation. Think hard before starting to program.
Use feedback judiciously use alpha and beta versions do not be overconfident from an early success
Study failures understand that complex systems fail for complex reasons.
Lecture 5
Curbing complexity
In absence of physical laws curb the complexity by good judgment. Easier said than done because: tempted to add new features than in the previous generation competitors have already incorporated the new features the features seem easy to implement the technology has improved human behavior: arrogance, pride, overconfidence…
Lecture 5
MICROPROCESSORS (1980s)MULTI-CORE MICROPROCESSORS
(2000s)
WORLD WIDE WEB (1990s)
GOOGLE, YouTube (2000s)
FIBER OPTICS (1990s)
WIRELESS (2000s)
SENSORSDIGITAL CAMERAS
(2000s)
COLLECT
PROCESSDISSEMINATE
COMMUNICATE
OPTICAL STORAGEHIGH DENSITY SOLID-STATE
(1990s)SPINTRONICS (2000s)
MILESTONES IN INFORMATION PROCESSING
BOOLEAN ALGEBRA (1854)DIGITAL COMPUTERS (1940s)INFORMATION THEORY (1948)
- Quantum Computing- Quantum Information Theory
STORE
Critical elements of information revolution!
Lecture 5
Complexity of computing and communication systems
New components
New applications
Interconnectivity + mobility, embedded
devices
Physical constraints
Larger segment of population using
the systems
Optimization of resource
consumption
TimingConstraints
Lecture 5
Names and fundamental abstractions
The fundamental abstractions1. Storage mem, disk, data struct, File Systems, disk arrays
2. Interpreters cpu, programming language e.g. java VM
3. Communication wire, Ethernet
rely on names. Naming:
Flat Hierarchical
Lecture 5
Lecture 5 16
Names and the three basic abstractions
Memory stores named objects write(name, value) value READ(name) file system: /dcm/classes/Fall09/Lectures/Lecture5.ppt
Interpreters manipulates named objects machine instructions ADD R1,R2 modules Variables call sort(table)
Communication Links connect named objects HTTP protocol used by the Web and file systems
Host: boticelli.cs.ucf.edu
put /dcm/classes/Fall09/Lectures/Lecture5.ppt
get /dcm/classes/Fall09/Lectures/Lecture5.ppt
Lecture 5
Latency and Bandwidth
Important concepts for physical characterization. Applies to all three abstractions. Informal
Bandwidth number of operations per second! Latency to get there
The bandwidth of the CPU, Memory, and I/O sbsystems must be balanced.
17
Lecture 5 18
Communication latency- time it takes the first bit sent to reach the receiver
Latency
Bandwidth
Bandwidth- number of bits/bytes transmitted per unit of time
t1
t2
t3
t4
t5
t6
Time Time
Sender Receiver
Program Storagedevice
t2t3
t1
Operation latency- time it takes the command to read the device
Latency
Bandwidth- number of bits/bytes transmitted per unit of time
Bandwidth
t4
t5
t6
Lecture 5 19
Memory Hardware memory:
Devices RAM (Random Access Memory) chip Flash memory non-volatile memory that can be erased and
reprogrammed Magnetic tape Magnetic Disk CD and DVD
Systems RAID File systems DBMS (Data Base management Systems)
Lecture 5 20
Attributes of the storage medium/system
Durability the time it remembers Stability whether or not the data is changed during the
storage Persistence property of data storage system, it keeps
trying to preserve the data
Lecture 5 21
Critical properties of a storage medium/system
Read/Write Coherence the result of a READ of a memory cell should be the same as the most recent WRITE to that cell.
Before-or-after atomicity the result of every READ or WRITE is as if that READ or WRITE occurred either completely before or completely after any other READ or WRITE
Lecture 5 22
time
WRITE item A in cell M
AM
READ from cell M
AM
A A
Read/Write Coherence è the result of a READ of a memory cell should be the same as the most recent WRITE to that cell.
Before-or-after atomicity è the result of every READ or WRITE is as if that READ or WRITE occurred either completely before or completely after any other READ or WRITE
Current READ/WRITE
Previous READ/WRITE
Next READ/WRITE
Lecture 5 23
Why it is hard to guarantee the critical properties?
Concurrency multiple threads could READ/WRITE to the same cell
Remote storage The delay to reach the physical storage may not guarantee FIFO operation
Optimizations data may be buffered to increase I/O efficiency
Cell size may be different than data size data may be written to multiple cells.
Replicated storage difficult to maintain consistency.
Lecture 5 24
Access type; access time
Sequential access Tapes CD/DVD
Random access devices Disk
Seek Search time Read/Write time
RAM
Lecture 5
Physical memory organization
RAM two dimensional array. To select a flip-flop provide the x and y coordinates.
Tapes blocks of a given length and gaps (special combination of bits.
Disk: Multiple platters Cylinders correspond to a particular position of the moving arm Track circular pattern of bits on a given platter and cylinder Record multiple records on a track
25
Lecture 5
Names and physical addresses
Location addressed memory the hardware maps the physical coordinates to consecutive integers, addresses
Associative memory unrestricted mapping; the hardware does not impose any constraints in mapping the physical coordinates
26
Figure 2.2 from textrbook
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